Self-suspending proppants for hydraulic fracturing

ABSTRACT

The present invention provides modified proppants, and methods for their manufacture. In embodiments, the modified proppant comprises a proppant particle and a hydrogel coating, wherein the hydrogel coating is applied to a surface of the proppant particle and localizes on the surface to produce the modified proppant. In embodiments, formulations are disclosed comprising the modified particles, and methods are disclosed for using the formulations.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 61/529,600, filed Aug. 31, 2011, U.S. Provisional Application Ser. No. 61/635,612 filed Apr. 19, 2012, and U.S. Provisional Application Ser. No. 61/662,681, filed Jun. 21, 2012. The entire contents of the above-referenced applications are incorporated by reference herein.

FIELD OF APPLICATION

This application relates generally to systems and methods for fracturing technologies.

BACKGROUND

In the process of acquiring oil and/or gas from a well, it is often necessary to stimulate the flow of hydrocarbons via hydraulic fracturing. The term “fracturing” refers to the method of pumping a fluid into a well until the pressure increases to a level that is sufficient to fracture the subterranean geological formations containing the entrapped materials. This process results in cracks and breaks that disrupt the underlying layer to allow the hydrocarbon product to be carried to the well bore at a significantly higher rate. Unless the pressure is maintained, however, the newly formed openings close. In order to open a path and maintain it, a propping agent or “proppant” is injected along with the hydraulic fluid to create the support needed to preserve the opening. As the fissure is formed, the proppants are delivered in a slurry where, upon release of the hydraulic pressure, the proppants form a pack or a prop that serves to hold open the fractures.

To accomplish the placement of the proppants inside the fracture, these particles are suspended in a fluid that is then pumped to its subterranean destination. To prevent the particles from settling, a high viscosity fluid is often required to suspend them. The viscosity of the fluid is typically managed by addition of synthetic or naturally-based polymers. There are three common types of polymer-enhanced fluid systems in general use for suspending and transporting proppants during hydraulic fracturing operations: slickwater, linear gel, and crosslinked gel.

In slickwater systems, an anionic or cationic polyacrylamide is typically added as a friction reducer additive, allowing maximum fluid flow with a minimum of pumping energy. Since the pumping energy requirements of hydraulic fracturing are high, on the order of 10,000-100,000 horsepower, a friction reducer is added to slickwater fluids to enable high pumping rates while avoiding the need for even higher pumping energy. While these polymers are effective as friction reducers, they are not highly effective as viscosifiers and suspending agents. Slickwater polymer solutions typically contain 0.5-2.0 gallons of friction reducer polymer per 1000 gallons of slickwater fluid, and the solutions have low viscosity, typically on the order of 3-15 cps. At this low viscosity, suspended proppant particles can readily settle out of suspension as soon as turbulent flow is stopped. For this reason, slickwater fluids are used in the fracturing stages that have either no proppant, proppant with small particle size, or low proppant loadings.

The second type of polymer enhanced fluid system is known as a linear gel system. Linear gel systems typically contain carbohydrate polymers such as guar, hydroxyethylcellulose, hydroxyethyl guar, hydroxypropyl guar, and hydroxypropylcellulose. These linear gel polymers are commonly added at a use rate of 10-50 pounds of polymer per 1000 gallons of linear gel fluid. These concentrations of linear gel polymer result in a fluid with improved proppant suspending characteristics vs. the slickwater fluid. The linear gel fluids are used to transport proppants, at loading levels of about 0.1 to 1 pound of proppant per gallon of fluid. Above this proppant loading level, a more viscous solution is typically required to make a stable suspension.

Crosslinked gel is the most viscous type of polymer-enhanced fluid used for transporting of proppant. In crosslinked gel systems, the linear gel fluid as described above is crosslinked with added reagents such as borate, zirconate, and titanate in the presence of alkali. Upon crosslinking of the linear gel fluid into a crosslinked gel fluid, the viscosity is much higher and the proppants can be effectively suspended. The linear gel and crosslinked gel fluids have certain advantages but they require a high dose rate of expensive polymer.

Modifications of proppant particles could be used advantageously to improve their performance in hydraulic fracturing systems. First, if the proppant particles were more buoyant, a less viscous suspension fluid could be used, which would still convey the particles to the target area but which would be easier to pump into the formation. Second, it is desirable that the proppants remain where they are placed throughout the lifetime of the well after they have been injected into a fracture line. If changes within the reservoir during well production force the proppants out of position, production equipment can be damaged, and the conductivity of the reservoir formation can be decreased as the reservoir pores are plugged by the displaced proppants. Third, the proppants in the system should be resistant to closure stress once they are placed in the fracture. Closure stresses can range from 1700 psi in certain shale gas wells, up to and exceeding 15,000 psi for deep, high temperature wells. Care must be taken that the proppants do not fail under this stress, lest they be crushed into fine particles that can migrate to undesirable locations within the well, thereby affecting production. Desirably, a proppant should resist diagenesis during fracture treatment. The high pressures and temperatures combine with the chemicals used in frac fluids can adversely affect the proppant particles, resulting in their diagenesis, which can eventually produce fine particulate matter that can scale out and decrease the productivity of the well over time.

Current proppant systems and polymer-enhanced fracturing fluids endeavor to address these concerns, so that the proppants can be carried by the fracturing fluids, can remain in place once they arrive at their target destination, and can resist the closure stresses in the formation. One approach to preparing suitable proppants includes coating the proppant materials with resins. A resin-coated proppant can be either fully cured or partially cured. The fully cured resin can provide crush resistance to the proppant substrate by helping to distribute stresses among the grain particles. A fully cured resin can furthermore help reduce fine migration by encapsulating the proppant particle. If initially partially cured, the resin may become fully cured once it is placed inside the fracture. This approach can yield the same benefits as the use of a resin that is fully-cured initially. Resins, though, can decrease the conductivity and permeability of the fracture, even as the proppants are holding it open. Also, resins can fail, so that their advantages are lost. Resin-based systems tend to be expensive and they are still prone to settling out of suspension.

Another approach to preparing suitable proppants involves mixing additives with the proppant itself, such as fibers, elastomeric particles, and the like. The additives, though, can affect the rheological properties of the transport slurry, making it more difficult to deliver the proppants to the desired locations within the fracture. In addition, the use of additives can interfere with uniform placement of the proppant mixture into the fracture site.

In addition, there are health, safety and environmental concerns associated with the processing of proppants. For example, fine particulates (“fines”), such as crystalline silica dust, are commonly found in naturally occurring sand deposits. These fines can be released as a respirable dust during the handling and processing of proppant sand. With chronic exposure, this dust can be harmful to workers, resulting in various inhalation-associated conditions such as silicosis, chronic obstructive pulmonary disease, lung cancers in the like. In addition to these health effects, the fines can cause “nuisance dust” problems such as fouling of equipment and contamination of the environment.

While there are known methods in the art for addressing the limitations of proppant systems, certain problems remain. There is thus a need in the art for improved proppant systems that allow precise placement, preserve fracture conductivity after placement, protect well production efficiency and equipment life, and promote worker health and safety. It is further desirable that such improved systems be cost-effective.

SUMMARY

Disclosed herein, in embodiments, are modified proppants, comprising a proppant particle and a hydrogel coating, wherein the hydrogel coating is applied to a surface of the proppant particle and localizes on the surface to produce the modified proppant. The hydrogel coating can comprise a water-swellable polymer. In embodiments, the hydrogel coating is applied to the surface as a liquid, which can comprise a solvent or a carrier fluid; the liquid hydrogel coating can become a dried hydrogel coating by removal of the solvent or the carrier fluid. In embodiments, the hydrogel coating comprises a water-swellable polymer that responds to elevated temperatures or brine conditions by collapsing in volume or thickness. In embodiments, the hydrogel coating comprises a hydrophobic comonomer selected from the group consisting of alkyl acrylate esters, N-alkyl acrylamides, N-isopropylacrylamide, propylene oxide, styrene, and vinylcaprolactam. In embodiments, the dried hydrogel coating is capable of expanding in volume in contact with an aqueous fluid to form a swollen hydrogel coating having a thickness of at least about 10% greater than the dried hydrogel coating. In embodiments, the hydrogel coating comprises a polymer selected from the group consisting of polyacrylamide, poly(acrylic acid), copolymers of acrylamide with acrylic acid salts, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, guar gum, carboxymethyl guar, carboxymethyl hydroxypropyl guar gum, hydrophobically associating swellable emulsion polymers, and latex polymers. In embodiments, the hydrogel coating further comprises chemical additives selected from the group consisting of scale inhibitors, biocides, breakers, wax control agents, asphaltene control agents, and tracers.

In embodiments, the modified proppant further comprises a cationic/anionic polymer pair comprising a cationic polymer and a high molecular weight anionic polymer; the cationic polymer can be selected from the group consisting of poly-DADMAC, LPEI, BPEI, chitosan, and cationic polyacrylamide. In embodiments, the modified proppant further comprises a crosslinking agent; the crosslinking agent can comprise a covalent crosslinker, and the covalent crosslinker can comprise a functional group selected from the group consisting of an epoxide, an anhydride, an aldehyde, a diisocyanate, and a carbodiamide. In embodiments, the covalent crosslinker can be selected from the group consisting of polyethylene glycol, diglycidyl ether, epichlorohydrin, maleic anhydride, formaldehyde, glyoxal, glutaraldehyde, toluene diisocyanate, and methylene diphenyl diisocyanate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiamide. In embodiments, the modified proppant can further comprise a delayed hydration additive; the delayed hydration additive can be selected from the group consisting of a low hydrophilic-lipophilic balance surfactant, an exclusion agent capable of excluding a finishing surfactant, a light ionic crosslinking agent, a light covalent crosslinking agent and a monovalent salt charge shielder. In embodiments, the modified proppant further comprises an alcohol selected from the group consisting of ethylene glycol, propylene glycol, glycerol, propanol, and ethanol. In embodiments, the modified proppant further comprises an anticaking agent.

Also disclosed herein are hydraulic fracturing formulations comprising the modified proppant described above. In embodiments, the formulations can further comprise uncoated sand and/or fibers. Methods are disclosed herein, in embodiments, for fracturing a well, comprising preparing the hydraulic fracturing formulation described above, and introducing the hydraulic fracturing formulation into the well in an effective volume and at an effective pressure for hydraulic fracturing, thereby fracturing the well.

Also disclosed herein, in embodiments, are methods of forming a modified proppant, comprising providing a proppant particle; and applying a hydrogel coating to a surface of the proppant particle so that the hydrogel coating localizes on the surface. In embodiments, the hydrogel coating is applied to the surface as a liquid. The methods can further comprise comprising the step of drying the hydrogel coating on the surface by a drying process, which can comprise heating the hydrogel coating. In embodiments, the hydrogel coating comprises a solvent or a carrier fluid, and the hydrogel coating dries on the surface by removal of the solvent or the carrier fluid to form a dried hydrogel coating. In embodiments, the method can comprise the further step of exposing the dried hydrogel coating to an aqueous fluid to form a swollen hydrogel coating, wherein the swollen hydrogel coating expands in volume to have a thickness of at least about 10% greater than the thickness of the dried hydrogel coating.

In addition, methods are disclosed herein for manufacturing a modified proppant, comprising providing a proppant substrate particle and a fluid polymeric coating composition, applying the fluid polymeric coating composition on the proppant substrate particle, mixing the proppant substrate particle and the fluid polymer coating composition to form a modified proppant, and drying the modified proppant, wherein the fluid polymeric coating composition comprises a hydrogel polymer, and wherein the hydrogel polymer localizes on the surface of the proppant substrate particle to produce the modified proppant. In embodiments, the manufacturing takes place at or near a point of use for the modified proppant. In embodiments, the proppant substrate particle comprises sand. In embodiments, the sand is obtained at or near the point of use for the modified proppant. These methods can further comprise adding an alcohol selected from the group consisting of ethylene glycol, propylene glycol, glycerol, propanol, and ethanol during or before the step of mixing the proppant substrate particles and the fluid polymer coating composition. These methods can further comprise adding an inversion promoter during or following the step of mixing the proppant substrate particles and the fluid polymer coating composition. These methods can further comprise adding an anticaking agent to the modified proppant.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow diagram of a manufacturing process for self-suspending proppants.

FIG. 2 is a graph of bed height vs. shear time for three sets of self-suspending proppant samples.

FIG. 3 is a graph of bed height vs. mixing time for two sets of self-suspending proppant samples.

FIG. 4 is a graph of bed height vs. mixing time for two sets of self-suspending proppant samples.

FIG. 5 is a graph of bed height vs. mixing time for a series of treated self-suspending proppant samples.

FIG. 6 is a graph of bed height for varying amounts of calcium silicate added to self-suspending proppant samples.

FIG. 7 is a graph of bed height vs. drying time for a series of preheated and non-preheated proppant samples.

FIG. 8 shows a graph of bed height vs. drying time at various temperatures.

FIG. 9 shows a graph of temperature vs. mixing time for a series of treated self-suspended proppant samples.

FIG. 10 shows a graph of bed height and loss of ignition (LOI) vs. drying time.

DETAILED DESCRIPTION 1. Modified Proppant Particles

Disclosed herein are systems and methods for forming and using proppant particles having a hydrogel surface layer to enhance the hydrodynamic volume of the proppant particles during fluid transport, creating a more stable proppant suspension that resists sedimentation, separation, and screenout before the proppant can reach the intended target destination in the fracture. Further benefits of the hydrogel-coated proppants as disclosed herein include lower tendency to erode equipment, lower friction coefficient in the wet state, good bonding adhesion with each other after placement in a fracture site, resistance to uncontrolled fines formation, and anti-fouling properties attributable to the hydrophilic surface. In embodiments, the disclosed systems for forming proppant particles can be applied to the types of proppant substrates most widely used, e.g., sand, resin coated sand, and ceramics. In other embodiments, the proppant particles can be formed from a variety of substrates, including fibrous materials, as would be available to those having ordinary skill in the art. In certain embodiments, the proppant particles can be fabricated so that they resist crush or deformation, so that they resist displacement, and so that they can be suspended in less viscous fluid carriers for transporting into the formation.

The invention encompasses a modified proppant, comprising a proppant particle and a hydrogel coating, wherein the hydrogel coating localizes on the surface of the proppant particle to produce the modified proppant. In embodiments, these self-suspending proppants are formed by modification of a particulate substrate with a water swellable polymer coating such as a hydrogel. In embodiments, the particulate substrate can be modified with the polymer coating before the particulate substrate is introduced into the fracturing fluid. In embodiments, the amount of hydrogel polymer coating can be in the range of about 0.1 to about 10% based on the weight of the proppant. In embodiments, the hydrogel layer applied onto the surface of the proppant substrate can be a coating thickness of about 0.01% to about 20% of the average diameter of the proppant substrate. Upon hydration and swelling of the hydrogel layer in the fracturing fluid, the hydrogel layer can become expanded with water, such that the hydrogel layer thickness can become about 10% to about 1000% of the average diameter of the proppant substrate.

Methods for modification of proppant include spraying or saturation of a liquid polymer formulation onto a proppant substrate, followed by drying to remove water or other carrier fluids. The drying process can be accelerated by application of heat or vacuum, and by tumbling or agitation of the modified proppant during the drying process. The heating can be applied by forced hot air, convection, friction, conduction, combustion, exothermic reaction, microwave heating, or infrared radiation. Agitation during the proppant modification process has a further advantage of providing a more uniform coating on the proppant material.

FIG. 1 illustrates schematically a manufacturing process 100 for preparing self-suspending proppant 130 in accordance with the present disclosure. In the depicted embodiment, sand 132 (e.g., dry sand having less than 0.1% moisture) is conveyed via a conveyor belt 122 into a mixing vessel 124, and a liquid polymer composition 120 is sprayed via pump and spray nozzle apparatus 134 onto the sand 132 along the conveyor belt 122. The sand 132 exposed to the liquid polymer 120 reports to a low shear mixing vessel 124, where the ingredients are further blended to form modified sand 128. After mixing, the modified sand containing the liquid polymer is sent to a dryer 126 to remove water and/or organic carrier fluids associated with the liquid polymer 120. After the drying step, the dried modified sand 132 is passed through a finalizing step 134, which can include a shaker and/or other size classification equipment such as a sieve to remove over-sized agglomerates. The finalizing step 134 can also subject the dried modified sand 132 to mechanical mixers, shear devices, grinders, crushers or the like, to break up aggregates to allow the material to pass through the appropriate sized sieve. The finished material 130 is then stored for shipment or use.

In embodiments, the sand that is used to produce self-suspending proppant is pre-dried to a moisture content of <1%, and preferably <0.1% before being modified with a hydrogel polymer. In embodiments, the sand temperature at the time of mixing with the liquid polymer is in the range of about 10 to about 200 degrees C., and preferably in the range of about 15 to about 80 degrees C.

In embodiments, the sand is contacted with the liquid polymer composition by means of spraying or injecting. The amount of liquid polymer composition added is in the range of about 1 to about 20%, and preferably about 2 to about 10% by weight of the sand. The sand and liquid polymer are blended for a period of 0.1 to 10 minutes. In a preferred embodiment, the mixing equipment is a relatively low shear type of mixer, such as a tumbler, vertical cone screw blender, v-cone blender, double cone blender, pug mill, paddle mixer, or ribbon blender. In embodiments, the mixing equipment can be equipped with forced air, forced hot air, vacuum, external heating, or other means to cause evaporation of the carrier fluids.

In embodiments, the modified sand containing the liquid polymer is dried to remove water and/or organic carrier fluids associated with the liquid polymer. The dryer equipment can be a conveyor oven, microwave, or rotary kiln type. In an embodiment the drying step is carried out in such a way that the dried, modified sand contains less than 1% by weight of residual liquids, including water and any organic carrier fluids associated with the liquid polymer composition.

In embodiments, the same equipment can be used to blend the sand with the liquid polymer and to dry the blended product in a single processing stage, or in a continuous production line. In an embodiment, the process of converting a substrate such as sand into a self-suspending proppant can be conducted at or near the point of use, for example at an oil or gas well site in preparation for hydraulic fracturing. This method has the advantage of converting a commodity material with high material handling costs, such as sand, into a specialized material that has added features. The sand can be acquired from local sources or shipped directly from a sand mining site or warehouse, for modification at the point of use. This avoids having to ship sand first into a blending plant and then ship a second time from the blending plant to the point of use. In the case of sand, the shipping costs can be higher than the material costs, so avoidance of extra shipping is desirable for controlling costs.

In an exemplary manufacturing process, the sand and the modifying chemicals can be added to a continuous mixer. After mixing is complete, the mixture can either be (a) ready to use or (b) sent to a drying step. The drying step can include a thermal or vacuum drying process, and it can include the addition of anticaking agents. The finished product can be stored in containers at the well site. An example of the mixing equipment is a continuous ribbon blender or a pug mill. The drying step can be a separate process from mixing, and the drying step can be designed to avoid overshearing of the finished product such as a conveyor or tunnel dryer. Other types of drying mechanisms include rotary kilns, microwave driers, paddle driers, and vacuum driers.

Hydrogel polymers that can be used to modify proppants in accordance with the systems and methods disclosed herein can be introduced, in embodiments, as oil-based emulsions, suspensions, water-based emulsions, latexes, solutions, and dispersions. In embodiments, the hydrogel polymers can be introduced as a distilled emulsion, such as an oil based emulsion that has been partially evaporated to remove a portion of the carrier fluids. This can offer the advantage of reduced drying requirements compared with conventional emulsions. In embodiments, the hydrogel polymer can be an alkali-swellable emulsion, wherein the hydrogel properties of the polymer are not fully developed until the polymer is contacted with alkali. In this embodiment, the alkali-swellable emulsion can be coated onto the proppant substrate to form a modified proppant, and this modified proppant can be suspended in a fracturing fluid in the presence of an alkaline material.

In embodiments, an additive such as an alcohol selected from the group consisting of ethylene glycol, propylene glycol, glycerol, propanol, and ethanol can be added during or before the step of mixing the proppant substrate particles and the liquid polymer coating composition. In embodiments, inversion promoters useful as additives in the polymer coating formulations for self-suspending proppants can include high HLB surfactants, such as polyethylene oxide lauryl alcohol surfactant, (ETHAL LA-12/80% from ETHOX), ethylene glycol, propylene glycol, water, sodium carbonate, sodium bicarbonate, ammonium chloride, urea, barium chloride, and mixtures thereof.

In other embodiments, the proppant substrate can be modified with a polymer formulation, without the need for a drying step. This can be accomplished by the use of a solvent-free polymer formulation, or a curable formulation. In certain simplified methods, a dry or liquid polymer formulation can be applied onto the proppant substrate via inline mixing, and the modified material thus prepared can be used without further processing. The moisture content of the proppant substrate can be modified by addition or removal of water, or addition of other liquids, to allow the substrate to be effectively coated, handled, and delivered into the fracturing fluid.

The modified proppants can be further modified with a wetting agent such as a surfactant or other hydrophilic material to allow for effective dispersion into a fracturing fluid. When the hydrogel-modified proppants are suspended into a fracturing fluid, they are considered to be self-suspending if they require a lower viscosity fluid to prevent the particles from settling out of suspension.

The modified proppants can be further modified with an anticaking agent such as calcium silicate, magnesium silicate, colloidal silica, calcium carbonate, or microcrystalline cellulose to improve the flowability and handling properties of the modified proppant material.

The hydrogel-modified proppants of the invention can advantageously use a localized polymer concentration on the proppant surface, in contrast to the traditional approach of making the entire fluid medium viscous. This localized hydrogel layer can permit a more efficient use of polymer, such that a lower total amount of polymer can be used to suspend proppant, as compared, for example, with conventional polymer-enhanced fracturing fluids such as slickwater, linear gel, and crosslinked gel. Although the hydrogel-modified proppants are considered to be self-suspending, they can be used in combination with friction reducers, linear gels, and crosslinked gels.

The hydrogel-modified proppants as disclosed herein can have the advantage of delivering friction-reducing polymer into the fracturing fluid, so that other friction reducer polymers might not be required or might be required in lesser amounts when the hydrogel-modified proppants are used in hydraulic fracturing operations. In embodiments, some of the hydrogel polymer can desorb from the surface of the proppant to deliver friction reducing benefits or viscosity features to the fracturing fluid.

The hydrogel polymer used for preparation of hydrogel-modified proppants can, in embodiments, comprise polymers such as a polyacrylamide, copolymers of acrylamide with anionic and cationic comonomers, copolymers of acrylamide with hydrophobic comonomers, poly(acrylic acid), poly(acrylic acid) salts, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, guar gum, alginate, carrageenan, locust bean gum, carboxymethyl guar, carboxymethyl hydroxypropyl guar gum, hydrophobically associating swellable emulsion (HASE) polymers, latex polymers, starches, and the like. In embodiments, the hydrogel polymer can be crosslinked to enhance the water absorbing and swelling properties of the polymer. The crosslinkers can be introduced as an element of the hydrogel base polymer, or they can be introduced as chemical modifiers for pre-formed polymers.

Localizing the polymer around the proppant surface as described herein can result in a more effective use of polymer and can prevent proppant from settling out of a polymer solution. In embodiments, the polymer layer hydrates around the proppant effectively preventing proppant/proppant (interparticle) contact. This can prevent the proppant from forming a compact settled bed and can result in a proppant that is easier to resuspend in a fracturing fluid. The resuspension properties for the modified proppants can be important if the fluid flow is interrupted during hydraulic fracturing operations. In this event, when the flow is resumed it is important that the proppant can be resuspended to avoid the loss of proppant or the unintended blockage of a fluid path.

The polymer surface modifications as described herein can cause an increase in the effective hydrodynamic radius of the proppant particle when the polymer swells. This can result in increased drag on the proppant as well as effectively changing the overall hydrogel/particle density. Both can result in a proppant particle with a slower settling rate and superior transport properties.

In embodiments, polymer pairing or ionic crosslinking can be used to improve the hydrogel polymer retention on the surface of the proppant particles. For example, a cationic polymer can be deposited onto the proppant as a first layer to “lock in place” a second layer containing a hydrogel such as a high molecular weight anionic polymer. In embodiments, the cationic polymer can be polydiallyldimethylammonium chloride (poly-(DADMAC)), linear polyethylenimine (LPEI), branched polyethylenimine (BPEI), chitosan, epichlorohydrin/dimethylamine polymer, ethylene dichloride dimethylamine polymer, or cationic polyacrylamide. The cationic polymer layer can be placed on the proppant either before or after proppant surface modification with the anionic hydrogel layer. The ionic interaction can act as a crosslink mechanism to help prevent the anionic polymer from desorbing in high shear environments such as going through a pump or during pumping down the wellbore. The cationic polymer can also improve polymer retention by causing a delay in the hydration and extension of the anionic polymer chains. It is believed that less polymer chain extension during the pumping process will yield higher polymer retention on the proppant (i.e. less desorption).

Covalent crosslinking of the hydrogel polymer layer on proppant surface can improve the swelling properties of the polymer and the shear tolerance to prevent premature release of the hydrogel from the proppant. Covalent crosslinkers can include the following functional groups: epoxides, anhydrides, aldehydes, diisocyanates, carbodiamides, divinyl, or diallyl groups. Examples of these covalent crosslinkers include: PEG diglycidyl ether, epichlorohydrin, maleic anhydride, formaldehyde, glyoxal, glutaraldehyde, toluene diisocyanate, methylene diphenyl diisocyanate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiamide, and methylene bis acrylamide. Covalent crosslinking of the hydrogel polymer layer on the proppant surface can effectively create a swellable “polymer cage” around the proppant. The covalent bonds prevent the polymer from completely desorbing into solution. The slightly insoluble polymer layer is able to swell and produce a hydrated polymer layer.

Delayed/controlled hydration of polymer layer may be desirable to delay the hydration of the polymer surface modification during handling of the proppant and initial pump-down through the wellbore. Environmental factors such as humidity and rain could cause premature hydration of the polymeric coating, which would make it difficult to effectively meter the proppant dose into the blender during a hydraulic fracturing operation. It is also believed that a fully hydrated polymer layer can be more prone to desorption under the high shear conditions associated with pumping of a fracturing fluid down the tubular. For these reasons, it may be advantageous to engineer a surface-modified proppant having slower or delayed hydration properties. In embodiments, delayed hydration can be achieved by addition of a low hydrophilic-lipophilic balance (HLB) surfactant, exclusion of a high HLB finishing surfactant, ionic crosslinking, covalent crosslinking, charge shielding using a monovalent salt, or by incorporation of a hydrophobic layer such as a fatty acid, or a fatty alcohol.

In embodiments, hydrophobic groups can be incorporated into the hydrogel polymer to allow for hydrophobic interactions. This method can improve the salt tolerance of the hydrogel layer, such that the hydrogel layer remains swellable even in an aqueous fluid that contains elevated salt concentrations.

Also disclosed herein is a method of fracturing a well using a hydrogel coated proppant in combination with non-hydrogel-coated proppant. For example, the hydrogel-coated proppant can serve as a suspending agent for the non-hydrogel-coated proppant.

Also disclosed herein is a method of improving well productivity by improved proppant placement using a hydrogel-coated proppant. The hydrogel-coated proppant can be more effectively transported into the far end of fractures to enable higher productivity of oil and gas from a well. Because the surface-modified proppants disclosed herein can be less inclined to settle out of the fluid and easier to resuspend and transport through the fracture, it is believed that proppant placement will be more effective. The ability to transport proppant further into fractures could significantly increase the effectiveness of a fracturing stimulation operation, resulting in a larger of volume of higher density fractures. These fracture channels can advantageously allow gas/condensate to more easily flow into the wellbore from the reservoir.

Also disclosed herein is an improved method of proppant placement using a low viscosity fluid. The surface modified proppants as disclosed herein utilize polymers more effectively to suspend/transport proppant particles. The surface modification renders the proppant self-suspending, thereby reducing or eliminating the need for highly viscous fluids/gels to transport proppant. Hence, lower viscosity fluids can be used in combination with the surface-modified proppant to transport proppant into fractures. This would advantageously simplify the formulation of fracturing gels for use with proppants.

Also disclosed herein is a more efficient method of fracturing a well using less proppant. Because highly effective proppant placement can be achieved with the easily-transportable surface-modified proppants as disclosed herein, it is anticipated that a smaller amount of these surface-modified proppants would be required for any given fracturing operation, as compared to systems using traditional proppants. With an increasing demand for fracturing grade sand/proppants, and a decreasing supply of desirably-shaped sand for proppant use, it would be advantageous to provide systems and methods such as those disclosed herein where less proppant can be used to achieve results comparable to or superior to the outcomes using current techniques.

After the hydrogel coated proppants of the invention have been pumped into a well, the hydrogel layer can be degraded by chemical, thermal, mechanical, and biological mechanisms. Specifically, the polymeric surface modification on the proppant can be broken down with the aid of chemical breakers, for example, ammonium persulfate, magnesium peroxide, or other oxidizers. The polymeric surface modification on the proppant can also be broken down with the aid of ambient reservoir conditions, such as elevated brine content, elevated temperature, and contact with hydrocarbons. Controlled breaking of the hydrogel layer upon reaching a target temperature or amount of time in the fluid, can be used as a means to direct the placement of the proppant in the desired location in fractures. The degradation of the hydrogel layer is also beneficial to ensuring the adequate conductivity of the propped fracture after completing the hydraulic fracturing operations. In embodiments, the hydrogel layer can demonstrate stimuli-responsive properties, so that it swells with water when exposed to a first set of conditions, such as a certain first temperature or pH, and it loses water, loses volume, loses thickness, or even collapses, when subjected to a certain set of conditions, such as a second temperature or pH.

For example, in an embodiment, temperature-responsive hydrogels can be coated onto proppant materials. The temperature responsive hydrogel layer can swell when exposed to water at a first set of conditions, such as a water temperature of 50-100 degrees F., and then it can collapse when exposed to a second set of conditions, such as a water temperature of 110-450 degrees F. Using this stimuli-responsive mechanism, the temperature responsive hydrogel coated proppant can have self-suspending properties as the fracturing fluid carries it underground to the location of the fractures at an initial water temperature, for example 50-100 degrees F. As the coated proppant encounters the higher temperature region of the underground formation, such as 110-450 degrees F., the hydrogel layer can collapse, allowing deposition and consolidation of the proppant in the fissures. The temperature responsive hydrogel can be a water soluble polymer or copolymer composition comprising hydrophobic monomers selected from the group consisting of alkyl acrylate esters, N-alkyl acrylamides, propylene oxide, styrene, and vinylcaprolactam. The N-alkyl substituted acrylamides can be N-isopropylacrylamide, N-butylacrylamide, N-octylacrylamide, and the like. The alkyl acrylate esters can be substituted by alkyl chains having from 1 to about 30 carbons. In a preferred embodiment, the temperature responsive hydrogel polymer comprises N-isopropylacrylamide and contains up to about 90 percent of hydrophilic comonomer units. The type and amount of the hydrophobic monomer substituent in the hydrogel polymer can be selected by experimental optimization techniques to adjust the water solubility and the temperature responsive properties of the hydrogel polymer.

Also disclosed herein is a method of delivery of chemical additives into the proppant pack, by incorporating the chemical additives into, or associated with, the hydrogel layer of the modified proppant. The chemical additives that can be advantageously delivered via the hydrogel layer include scale inhibitor, biocide, breaker, wax control, asphaltene control, and tracers. The chemical additives can be in the form of water soluble materials, water insoluble particles, fibers, metallic powders or flakes, and the like. The chemical additives can be selected such that they slowly dissolve or decompose to release their chemical activity. In embodiments, chemical additives can be incorporated into, or associated with, the hydrogel layer by physical entrainment, layer-by-layer deposition, covalent attachment, ionic association, hydrophobic association, or encapsulation. The chemical additives can be added to the proppant separately from the hydrogel, or they can be combined with the hydrogel coating formulation at the time of manufacture of the coated proppant. Breaker chemicals such as persulfates, peroxides, permanganates, perchlorates, periodates or percarbonates can be added in this method of delivery. The transport and delivery of these chemicals with the hydrogel coated proppant has the advantage of a targeted delivery of the chemicals to a fracture or to a proppant pack. This method offers the advantage of concentrating the chemical additives in the location where their function is needed, thereby reducing the overall amount of chemical additives needed. Certain breakers such as peroxides and persulfates have an accelerated activity at higher temperatures. Using this method, the breaker chemicals incorporated in the hydrogel layer will become activated upon placement in the fractures, by the elevated temperatures of the petroleum bearing reservoir.

In embodiments, the surface of a proppant particulate substrate can be coated with a selected polymer, either as a single layer or as a series of multiple coating layers. The coating (either single layer or multilayer) can show switchable behavior under certain circumstances. As used herein, the term “switchable behavior” or “switching behavior” refers to a change in properties with a change in circumstances, for example, a change from one set of properties during the transport phase and another set of properties inside the fracture. Switching behavior can be seen, for example, when a particle demonstrates hydrophilic properties in the fracturing fluid and adhesive properties when in place within the fractures. Such behavior can triggered by circumstances like the high closing pressures inside the fracture site so that the outer layer of the coating rearranges itself to exhibit more advantageous properties.

In an embodiment, the coated particle can switch from hydrophilic to hydrophobic when subjected to the high pressures inside the fractures. In an exemplary embodiment, during the transport phase, when the hydrophilic covering of the particle is exposed to the water-based fracturing fluid, it will tend to be fully distended. As a result, the coating can provide the particle with lubrication in this state, facilitating its movement through the proppant slurry. When the particle has been conveyed to its destination within the fractures in the formation though, the high pressures there will overcome the steric repulsions of the external hydrophilic polymer chains, forcing the outer layer to rearrange itself so that the inner layer is exposed. In embodiments, the switchable inner layer can be hydrophobic or adhesive, or both. As the inner layer becomes exposed, its properties can manifest themselves. If the inner layer has adhesive properties, for example, it can fix the particles to each other to prevent their flowback. This inner layer can also be configured to capture fines in case the proppant particle fails. Moreover, the residual intact hydrophilic groups present in the outer coating can allow easy flow of oil through the proppant pack.

In embodiments, a coated proppant particle can be produced that bears the following layers of coating. First, a pressure-activated fixative polymer can be used to coat the proppant substrate. This coating layer can be elastomeric, thereby providing strength to the proppant pack by helping to agglomerate the proppant particles and distribute stress. In addition, this coating layer can encapsulate the substrate particles and retain any fines produced in the event of substrate failure. Second, a block copolymer can be adsorbed or otherwise disposed upon the first layer of coating. The copolymer can have a section with high affinity for the first polymeric layer, allowing strong interaction (hydrophobic interaction), and can have another section that is hydrophilic, allowing for easy transport of the proppant in the transport fluid.

In certain embodiments, a stronger interaction between the first and second coating layers may be useful. To accomplish this, a swelling-deswelling technique can be implemented. For example, the block copolymer can be adsorbed onto the surface of the elastomeric-coated particle. Then, the first coating layer can be swelled with small amounts of an organic solvent that allow the hydrophobic block of the copolymer to penetrate deeper into the first coating layer and to become entangled in the elastomeric coating. By removing the organic solvent, the layered polymeric composite will deswell, resulting in a stronger interaction of copolymer with the elastomeric particle. A method for swelling-deswelling technique that can be useful is set forth in “Swelling-Based Method for Preparing Stable, Functionalized Polymer Colloids,” A. Kim et al., J. Am. Chem. Soc. (2005) 127: 1592-1593, the contents of which are incorporated by reference herein.

In embodiments, proppant systems using coatings as disclosed herein can decrease the amount of airborne particles associated with proppant manufacture. For example, respirable dust including fine crystalline silica dust associated with handling and processing proppant sand can be captured and held by the proppant coatings during their processing. In embodiments, coating agents can be added that have a particular affinity for particulates in the environment that could adversely affect worker safety or create nuisance dust problems. In embodiments, a hydrogel coating on proppant particles can serve as a binder or capturing agent by mechanically entrapping or adhering to the dust particulates.

While the systems described herein refer to a two-layer coating system, it is understood that there can be multiple (i.e., more than two) coating layers forming the composite proppant particles disclosed herein, with the each of the multiple coating layers possessing some or all of the attributes of the two coating layers described above, or with one or more of the multiple coating layers providing additional properties or features.

2. Particulate Substrate Materials

Composite proppant particles in accordance with these systems and methods can be formed using a wide variety of proppant substrate particles. Proppant particulate substrates can include for use in the present invention include graded sand, resin coated sand, bauxite, ceramic materials, glass materials, walnut hulls, polymeric materials, resinous materials, rubber materials, and the like, and combinations thereof. The self-suspending proppant (“SSP”) as disclosed herein can also be made using specialty proppants, such as ceramics, bauxite, and resin coated sand. By combining sand SSP with specialty SSP, a proppant injection can have favorable strength, permeability, suspension, and transport properties. In embodiments, the substrates can include naturally occurring materials, for example nutshells that have been chipped, ground, pulverized or crushed to a suitable size (e.g., walnut, pecan, coconut, almond, ivory nut, brazil nut, and the like), or for example seed shells or fruit pits that have been chipped, ground, pulverized or crushed to a suitable size (e.g., plum, olive, peach, cherry, apricot, etc.), or for example chipped, ground, pulverized or crushed materials from other plants such as corn cobs. In embodiments, the substrates can be derived from wood or processed wood, including but not limited to woods such as oak, hickory, walnut, mahogany, poplar, and the like. In embodiments, aggregates can be formed, using an inorganic material joined or bonded to an organic material. Desirably, the proppant particulate substrates will be comprised of particles (whether individual substances or aggregates of two or more substances) having a size in the order of mesh size 4 to 100 (US Standard Sieve numbers). As used herein, the term “particulate” includes all known shapes of materials without limitation, such as spherical materials, elongate materials, polygonal materials, fibrous materials, irregular materials, and any mixture thereof.

In embodiments, the particulate substrate can be formed as a composite from a binder and a filler material. Suitable filler materials can include inorganic materials such as solid glass, glass microspheres, fly ash, silica, alumina, fumed carbon, carbon black, graphite, mica, boron, zirconia, talc, kaolin, titanium dioxide, calcium silicate, and the like. In certain embodiments, the proppant particulate substrate can be reinforced to increase their resistance to the high pressure of the formation which could otherwise crush or deform them. Reinforcing materials can be selected from those materials that are able to add structural strength to the proppant particulate substrate, for example high strength particles such as ceramic, metal, glass, sand, and the like, or any other materials capable of being combined with a particulate substrate to provide it with additional strength.

In certain embodiments, the proppant particulate substrate can be fabricated as an aggregate of two or more different materials providing different properties. For example, a core particulate substrate having high compression strength can be combined with a buoyant material having a lower density than the high-compression-strength material. The combination of these two materials as an aggregate can provide a core particle having an appropriate amount of strength, while having a relatively lower density. As a lower density particle, it can be suspended adequately in a less viscous fracturing fluid, allowing the fracturing fluid to be pumped more easily, and allowing more dispersion of the proppants within the formation as they are propelled by the less viscous fluid into more distal regions. High density materials used as proppant particulate substrates, such as sand, ceramics, bauxite, and the like, can be combined with lower density materials such as hollow glass particles, other hollow core particles, certain polymeric materials, and naturally-occurring materials (nut shells, seed shells, fruit pits, woods, or other naturally occurring materials that have been chipped, ground, pulverized or crushed), yielding a less dense aggregate that still possesses adequate compression strength.

Aggregates suitable for use as proppant particulate substrates can be formed using techniques to attach the two components to each other. As one preparation method, a proppant particulate substrate can be mixed with the buoyant material having a particle size similar to the size of the proppant particulate substrates. The two types of particles can then be mixed together and bound by an adhesive, such as a wax, a phenol-formaldehyde novolac resin, etc., so that a population of doublet aggregate particles are formed, one subpopulation having a proppant particulate substrate attached to another similar particle, one subpopulation having a proppant particulate substrate attached to a buoyant particle, and one subpopulation having a buoyant particle attached to another buoyant particle. The three subpopulations could be separated by their difference in density: the first subpopulation would sink in water, the second subpopulation would remain suspended in the liquid, and the third subpopulation would float.

In other embodiments, a proppant particulate substrate can be engineered so that it is less dense by covering the surface of the particulate substrate with a foamy material. The thickness of the foamy material can be designed to yield a composite that is effectively neutrally buoyant. To produce such a coated proppant particulate, a particle having a desirable compression strength can be coated with one reactant for a foaming reaction, followed by exposure to the other reactant. With the triggering of foam formation, a foam-coated proppant particulate will be produced.

As an example, a water-blown polyurethane foam can be used to provide a coating around the particles that would lower the overall particle density. To make such a coated particle, the particle can be initially coated with Reactant A, for example a mixture of one or more polyols with a suitable catalyst (e.g., an amine). This particle can then be exposed to Reactant B containing a diisocyanate. The final foam will form on the particle, for example when it is treated with steam while being shaken; the agitation will prevent the particles from agglomerating as the foam forms on their surfaces.

In embodiments, fibers, including biodegradable fibers can be added to the fracture fluid along with SSP. Fibers, including biodegradable fibers, can form a fiber network that help carry the proppant with the fluid. A number of fiber types are familiar to skilled artisans for adding to fracture fluid. As would be understood by skilled artisans, fibers added to the fracture fluid can degrade under downhole conditions, and channels are formed in the proppant pack. The channels then have higher permeability and are therefore the flow channels through which hydrocarbons travel from the formation to the wellbore.

The term “fiber” can refer to a synthetic fiber or a natural fiber. As used herein, the term “synthetic fibers” include fibers or microfibers that are manufactured in whole or in part. Synthetic fibers include artificial fibers, where a natural precursor material is modified to form a fiber. For example, cellulose (derived from natural materials) can be formed into an artificial fiber such as Rayon or Lyocell. Cellulose can also be modified to produce cellulose acetate fibers. These artificial fibers are examples of synthetic fibers. Synthetic fibers can be formed from synthetic materials that are inorganic or organic. Exemplary synthetic fibers can be formed from materials such as substituted or unsubstituted lactides, glycolides, polylactic acid, polyglycolic acid, or copolymers thereof. Other materials to form fibers include polymers of glycolic acid or copolymers formed therewith, as are familiar to skilled artisans.

As used herein, the term “natural fiber” refers to a fiber or a microfiber derived from a natural source without artificial modification. Natural fibers include vegetable-derived fibers, animal-derived fibers and mineral-derived fibers. Vegetable-derived fibers can be predominately cellulosic, e.g., cotton, jute, flax, hemp, sisal, ramie, and the like. Vegetable-derived fibers can include fibers derived from seeds or seed cases, such as cotton or kapok. Vegetable-derived fibers can include fibers derived from leaves, such as sisal and agave. Vegetable-derived fibers can include fibers derived from the skin or bast surrounding the stem of a plant, such as flax, jute, kenaf, hemp, ramie, rattan, soybean fibers, vine fibers, jute, kenaf, industrial hemp, ramie, rattan, soybean fiber, and banana fibers. Vegetable-derived fibers can include fibers derived from the fruit of a plant, such as coconut fibers. Vegetable-derived fibers can include fibers derived from the stalk of a plant, such as wheat, rice, barley, bamboo, and grass. Vegetable-derived fibers can include wood fibers. Animal-derived fibers typically comprise proteins, e.g., wool, silk, mohair, and the like. Animal-derived fibers can be derived from animal hair, e.g., sheep's wool, goat hair, alpaca hair, horse hair, etc. Animal-derived fibers can be derived from animal body parts, e.g., catgut, sinew, etc. Animal-derived fibers can be collected from the dried saliva or other excretions of insects or their cocoons, e.g., silk obtained from silk worm cocoons. Animal-derived fibers can be derived from feathers of birds. Mineral-derived natural fibers are obtained from minerals. Mineral-derived fibers can be derived from asbestos. Mineral-derived fibers can be a glass or ceramic fiber, e.g., glass wool fibers, quartz fibers, aluminum oxide, silicon carbide, boron carbide, and the like.

Fibers may advantageously be selected or formed so that they degrade at specified pH or temperatures, or to degrade over time, and/or to have chemical compatibilities with specified carrier fluids used in proppant transport. Useful synthetic fibers can be made, for example, from solid cyclic dimers or solid polymers of organic acids known to hydrolyze under specific or tunable conditions of pH, temperature, time, and the like. Advantageously, fibers can decompose in the locations to which they have been transported under predetermined conditioned. Advantageously, the decomposition of the fibers can yield decomposition products that are environmentally benign.

EXAMPLES Materials

30/70 mesh frac sand

30/50 mesh frac sand

40/70 mesh frac sand

Polydiallyldimethylammonium chloride (Aldrich, St. Louis, Mo.)

LPEI 500 (Polymer Chemistry Innovations, Tucson, Ariz.)

Ethyl Alcohol, 200 Proof (Aldrich, St. Louis, Mo.)

Hexane (VWR, Radnor, Pa.)

FLOPAM EM533 (SNF)

Polyethyleneglycol diglycidyl ether (Aldrich, St. Louis, Mo.)

Glyoxal, 40 wt % solution (Aldrich, St. Louis, Mo.)

HFC-44 (Polymer Ventures, Charleston, S.C.)

Carboxymethyl Cellulose, sodium salt (Sigma-Aldrich, St. Louis, Mo.)

Ammonium Persulfate (Sigma-Aldrich, St. Louis, Mo.)

Ethoxylated lauryl alcohol surfactant (Ethal LA-12/80%)) (Ethox Chemical Co, SC)

Glycerol (US Glycerin, Kalamazoo, Mich.)

Potassium Chloride (Morton Salt, Chicago, Ill.)

Fumed Silica (Cabot, Boston, Mass.)

Example 1 Preparation of Inner Polymer Layer

An inner polymer layer of 100 ppm concentration was prepared on a sand sample by adding 200 g 30/70 mesh frac sand to a FlackTek Max 100 long jar. To the sand was added 85 g tap water and 2 g of a 1% polydiallyldimethylammonium chloride (PDAC) solution. The sample was then shaken by hand for approximately 5 minutes, vacuum filtered and dried in an oven at 80° C. The sand sample was then removed from the oven and used in subsequent testing.

An identical method was used as described above to formulate a 10 ppm inner polymer layer coating with the exception being that only 0.2 g of a 1% PDAC solution were used.

An identical method was used as described above to formulate an inner polymer layer at a maximum polymer loading (“Max PDAC”) with the exception that 1 g of a 20 wt % PDAC solution was used. Following treatment the sand was washed with excess tap water, vacuum filtered and dried in an oven at 80° C. The sand sample was then removed from the oven and used in subsequent testing.

Example 2 Preparation of Inner Polymer Layer

An inner polymer layer of 100 ppm concentration was prepared on a sand sample by dissolving 0.2 g LPEI 500 in 10 g ethanol to form a 2% LPEI 500 solution in ethanol. To 70 g ethanol in a 250 mL round bottom flask was added 0.75 g of the 2% LPEI 500 solution. Then 150 g of 30/70 mesh frac sand was added to the round bottom flask. The solvent was removed using a rotary evaporator with a 65° C. water bath. The sample was then removed from the flask and used in subsequent testing.

Example 3 Preparation of Outer Polymer Layer

Outer polymer layers were applied to sand samples by mixing sand with liquid Flopam EM533 polymer under different conditions. In one coating method, polymer product was added neat. In another coating method the polymer product was extended by diluting with hexane. For hexane dilution 10 g polymer was added to 10 g hexane in a 40 mL glass vial and vortex mixed until homogenous. Polymer was then added to 30/70 mesh frac sand samples of 30 g in FlackTek Max 100 jars. Samples were placed in a FlackTek DAC150 SpeedMixer at 2600 rpm for about 25 seconds. Samples were removed from SpeedMixer and allowed to dry in an oven at 80° C. overnight.

Example 4 Performance of Outer Polymer Layer, Settling Times

Sand samples prepared in previous example were assessed for performance in a settling test. Prior to testing, all sand samples were sieved through a 25 mesh screen. Settling times were obtained by adding 1 g of sand sample to 100 mL of tap water in a 100 mL graduated cylinder. The graduated cylinder was then inverted about 8 times and then the time required for all of the sand to settle at the bottom of the graduated cylinder was recorded. Three times were recorded for each sample. Settling times are reported in Table 1.

TABLE 1 Outer Treatment Settling Settling Settling Inner Layer Added Time1 Time2 Time3 Sample Layer Treatment (g) (sec) (sec) (sec) 1 100 ppm Flopam 1 34 35 32 PDAC EM533 2 100 ppm 50:50 2 25 25 26 PDAC Flopam EM533/ hexane 3 100 ppm Flopam 3 35 71 60 PDAC EM533 4 100 ppm 50:50 6 24 33 32 PDAC Flopam EM533/ hexane 5 Max Flopam 1 19 21 27 PDAC EM533 6 Max 50:50 2 17 23 21 PDAC Flopam EM533/ hexane 7 Max Flopam 3 29 31 35 PDAC EM533 8 Max 50:50 6 23 24 25 PDAC Flopam EM533/ hexane 9 None Flopam 1 22 22 22 EM533 10 None Flopam 3 25 54 64 EM533 11 None None 0 10 10 10

Example 5 Performance of Outer Polymer Layer, Settled Bed Height

Sand samples prepared in Example 3 with outer polymer layer were also assessed by observing the settled bed height in water. In a 20 mL glass vial, 1 g of a sand sample was added to 10 g tap water. The vials were inverted about 10 times to adequately wet the sand treatments. The vials were then allowed to sit undisturbed for about 30 minutes. A digital caliper was then used to record the height of the sand bed in the vial. Results are reported in Table 2.

TABLE 2 Sample 1 2 3 4 5 6 7 8 9 10 11 Bed 13.5 6.9 22.6 8.9 8.9 5.8 11.9 n/a 11.9 22.9 0.8 Height (mm)

Example 6 Ionic Crosslink of Outer Polymer Layer

A 40 g 30/70 mesh frac sand sample was treated with an outer polymer layer by adding 1.3 g Flopam EM533 polymer to the 40 g of sand in a FlackTek Max 100 jar and shaking the jar by hand for 2 minutes. The sand was then sieved through a 25 mesh screen. To assess polymer retention on sand under shear, tests were conducted by adding 10 g of treated sand to 200 g tap water with different levels of PDAC in a 300 mL glass beaker. It is believed that the PDAC will interact ionically to stabilize the polymer layer on the sand. The slurries were then stirred at 900 rpm with an overhead mixer using a flat propeller style mixing blade for 5 minutes. Mixing was then stopped and samples were allowed to settle for 10 minutes. Viscosity of the supernatant was then measured using a Brookfield DV-III+ rheometer with an LV-II spindle at 60 rpm. Bed height of the settled sand in the beaker was also recorded using a digital caliper. Results are reported in Table 3.

TABLE 3 Sample PDAC Conc. (ppm) Visc. (cP) Bed Height (mm) 12 0 25 4.5 13 60 10 8.6 14 200 2.5 6.3

Example 7 Covalent Crosslink of Outer Polymer Layer—PEGDGE

Four samples of 30/70 mesh frac sand were treated with Flopam EM533 by adding 0.66 g polymer to 20 g sand in a FlackTek Max 100 jar and shaking by hand for 2 minutes. Then various amounts of a fresh 1% polyethyleneglycol diglycidyl ether solution in deionized water were added to the treated sand samples. The samples were again shaken by hand for 2 minutes and then placed in an oven at 100° C. for 1 hour. Samples were then removed from the oven and sieved through a 25 mesh screen. Bed heights were measured for the four samples by adding 1 g of the sand sample to 10 g of tap water in a 20 mL glass vial, inverting the vials approximately 10 times to adequately wet the sand and allowing the vials to sit undisturbed for about 10 minutes. Bed heights were then measured with a digital caliper. Results are listed in Table 4.

TABLE 4 Sample 1% PEGDGE (g) Bed Height (mm) 15 0.1 9.3 16 0.2 8.8 17 1.0 6.2 18 0 12.7

Example 8 Covalent Crosslink of Outer Polymer Layer—Glyoxal

Four samples of 30/70 mesh frac sand were treated with Flopam EM533 by adding 0.66 g polymer to 20 g sand in a FlackTek Max 100 jar and shaking by hand for 2 minutes. A 1% glyoxal solution in ethanol was formulated by adding 0.25 g 40 wt % glyoxal to a 20 mL glass vial and diluting to 10 g with ethanol. Then varying amounts of the 1% glyoxal solution were added to the treated sand samples, and the samples were shaken by hand for 2 minutes and placed in the oven at 100° C. for 30 minutes. The sand samples were removed from the oven and sieved through a 25 mesh screen. For settled bed height measurements 1 g of sand was added to 10 g tap water in 20 mL vials, inverted about 10 times and given about 10 minutes to settle. Bed height was measured with a digital caliper. Results are listed in Table 5.

TABLE 5 Sample 1% glyoxal (g) Bed Height (mm) 19 0.2 3.8 20 0.5 3.5 21 1.0 2.7 22 2.0 2.7

Example 9 Cationic/Anionic Polymer Treatments

Three samples of 30 g of 30/70 mesh frac sand were treated with Polymer Ventures HCF-44 in a FlackTek Max 100 jar. The jar was shaken by hand for 2 minutes. Flopam EM533 was then added to each of the samples. The jars were again shaken by hand for 2 minutes. The samples were then dried at 80° C. overnight. The sand samples were removed from the oven and sieved through a 25 mesh screen. For settled bed height measurements 1 g of sand was added to 10 g tap water in 20 mL vials, inverted about 10 times and given about 10 minutes to settle. Bed height was measured with a digital caliper. Results are given in Table 6.

TABLE 6 Sample HCF-44 (g) Flopam EM533 (g) Bed Height (mm) 23 0 0.45 10.26 24 0.07 0.38 8.08 25 1.0 0.35 5.08 26 1.5 0.30 3.94

Example 10 Sand Coated with Macromolecular Particles

A 30 g sample of 30/70 mesh frac sand was added to a FlackTek Max 100 jar. To the sand, 0.3 g of paraffin wax was added. The sample was placed in a FlackTek DAC 150 SpeedMixer and mixed at 2500 rpm for 2 minutes. After mixing, 1 g of carboxymethyl cellulose was added to the sample. The sample was again placed in the FlackTek DAC 150 SpeedMixer and mixed at 2500 rpm for 1 minute. The sand sample was sieved through a 25 mesh screen. For settled bed height measurements 1 g of sand was added to 10 g tap water in a 20 mL vial, inverted about 10 times and given about 10 minutes to settle. The sand in this sample clumped together immediately and did not disperse in the water, and an accurate measurement of bed height could not be obtained.

Example 11 Modified Sand Beaker Testing

A 30 g sample of 30/70 mesh frac sand was added to a FlackTek Max 100 jar. The sand was treated with Flopam EM533 by adding 0.45 g of the polymer to the jar and shaking by hand for 2 minutes. The sample was then dried at 80° C. overnight. After drying, the sample was removed from the oven and sieved over a 25 mesh screen. After sieving, four samples were prepared by adding 1 g of the treated sand to 10 g of tap water in a 20 mL vial. The vials were inverted about 10 times and left to settle for 10 minutes. A 10% ammonium persulfate solution was made by adding 2 g of ammonium persulfate to 18 g of tap water. Varying amounts of the 10% ammonium persulfate solution were then added to the sample vials. The samples were inverted several times to mix, and then placed in an oven at 80° C. for 1 hr. After 1 hour the samples were removed and the settled bed heights were observed. Table 7 shows the results.

TABLE 7 Sample 10% APS (μL) Sand Suspension 27 0 Suspended 28 180 Settled 29 90 Settled 30 18 Settled

Example 12 Emulsion Additives

To determine the effect of emulsion additives on self-suspending proppant (“SSP”) performance, glycerol and Ethal LA-12/80% were added to the emulsion polymer EM533 before coating the proppant sand. Three different polymer samples were made as follows:

-   -   SSP Polymer: 10 g of EM533, no additive     -   SSP+glycerol: 9 g EM533 and 1 g of glycerol     -   SSP+glycerol+Ethal: 9 g EM533+0.9 g glycerol+0.1 g Ethal         LA-12/80%

Each of the above samples was vortex mixed for 30 seconds to ensure homogeneity. To make the modified proppant, 50 g of 40/70 sand was combined with 1.5 g of one of the polymer samples above and then mixed for 30 s. The modified proppant samples were evaluated for shear stability in the 1 liter shear test. This test involves addition of 50 grams of modified proppant to 1 liter of water in a square plastic beaker, followed by mixing on a paddle/jar mixer (EC Engineering model CLM-4) at 200 rpm (corresponding to a shear rate of about 550 s⁻¹) for different amounts of time. The sheared samples are then poured into a 1000 mL graduated cylinder and allowed to settle by gravity for 10 minutes, then the bed height of the settled proppant sand is recorded. For comparison, an unmodified proppant sand will produce a bed height of 10 mm after any amount of mixing. The self-suspending proppant samples will produce a higher bed level vs. unmodified proppant due to the hydrogel layer encapsulating the sand grains. Generally, increasing the shear rate or time can cause the bed height of self-suspending proppant to decrease as a result of desorption of the hydrogel layer from the surface of the modified proppant. For this reason, it is desirable for the bed height to be as high as possible in this test, especially after shear. The results below show that the addition of glycerol improves the bed height and the shear stability of the product. The addition of glycerol and Ethal, while it improves the initial bed height, the long term shear stability is slightly decreased. These results are illustrated in the graph in FIG. 2.

Example 13 Glycerol and Processability

This experiment sought to determine the effect of glycerol and other additives on the performance of self-suspending proppants (denoted as SSP below). 1 kg of dry 40/70 sand was added to the bowl of a KitchenAid stand mixer, model KSM90WH, which was fitted with the paddle attachment. 3.09 g of glycerol was mixed with 27.84 g of EM533 emulsion polymer, then the mixture was added to the top of the sand and allowed to soak in for 1 minute. At time 0 the mixer was started at speed 1 (72 rpm primary rotation). Samples were collected at 1-2 minute intervals and dried for 1 hour at 90° C. Then, each sample was subjected to the 1 liter shear test, where 50 g of SSP was added to 1 L of water and sheared at 200 rpm (an approximate shear rate of 550 s⁻¹) for 20 minutes. After transferring the water/SSP mixture to a 1 liter graduated cylinder and settling for 10 min, the bed heights were recorded. The experiment was repeated with 30.93 g EM533 emulsion polymer alone added to 1 kg of sand. These results are shown in FIG. 3. As shown in the graph, the glycerol additive increased bed heights significantly.

The difference in performance was even more marked when the experiment was repeated at higher mixing speeds. Here the mixer was set to speed 4 (150 rpm primary rotation). At low mixing times, the samples were insufficiently mixed, leading to incomplete coating of the sand and ready desorption of the polymer from the surface of the SSP during the shear test. As mixing time of the coating process increased so did performance, until an ideal coating was reached, giving maximum bed height for that sample. After that, increasingly worse (lower) bed heights were seen at higher mixing times, possibly as a result of abrasion of the coating during extended mixing. At higher mixing speeds, this process happened even faster, such that the processing window for the emulsion polymer alone was less than 1 minute. With the addition of glycerol and the use of a lower mixing speed, this processing window was widened to nearly 15 minutes. In comparison to the tests with emulsion polymer alone, glycerol caused the processing window to widen, indicating that SSP preparation with the glycerol is more robust. At the same time, glycerol allowed the polymer emulsion to invert more fully, leading to better coatings and increased bed heights. Testing with combinations of glycerol and the emulsion polymer EM533 at a higher mixing speed yielded the results shown in FIG. 4.

Example 14 Modified Proppant with an Anticaking Agent

Modified proppant samples were made with and without anticaking agent for a comparison. For Sample A, 50 g of 40/70 sand was added to a FlackTek jar. 1.5 g of EM533 emulsion polymer was added to the sand and the sample was mixed for 30 seconds. After mixing, 0.25 g of calcium silicate was added to the sample and the sample was mixed again for 30 seconds. The sample was then dried for 1 hour at 85° C. After drying, the sample was poured over a 25 mesh screen and shaken lightly for 30 seconds. The amount of sand that passed through the sieve was then measured. For Sample B, 50 g of 40/70 sand was added to a FlackTek jar. 1.5 g of EM533 emulsion polymer was added to the sand and the sample was mixed for 30 seconds. The sample was then dried for 1 hour at 85° C. After drying, the sample was poured over a 25 mesh screen and shaken lightly for 30 seconds. The amount of sand that passed through the sieve was then measured. Table 8 shows the results.

TABLE 8 Total Mass Mass passing % Passing Sample Sample, g Sieve, g Sieve Sample A 50.5 50.16 99.3% Sample B 50.5 15.71 31.1%

The results of sieve testing show that the incorporation of an anticaking agent was effective at improving the handling properties of the modified proppants.

Samples A and B were separately added to 1 L of water and then sheared in the EC Engineering Mixer for 20 minutes at 200 rpm. After shearing, the samples were transferred to a 1 L graduated cylinder and left to settle for 10 minutes. After settling, the bed heights were measured, showing no significant loss in shear stability as a result of incorporating an anticaking agent. Table 9 shows these results.

TABLE 9 Bed Height, Sample mm Sample A 56.21 Sample B 59.67

Example 15 Coating of Proppant with Hydrogel Layer

A coating composition was made by combining 10 g glycerol and 90 g Flopam EM533 in a glass vial and mixing for 30 seconds with a vortex mixer. Next, 400 g of 40/70 mesh proppant sand was added to a KitchenAid mixer bowl. 16 g of the coating composition was added to the KitchenAid mixer bowl. The mixer was then turned on to the lowest setting and left to mix for 7 minutes. After mixing, the sand was split into 50 g samples and placed in a forced air oven at 80° C. for 1 hr. After drying, the modified proppant was screened through a 25 mesh sieve.

Example 16 Coating of Proppant with Hydrogel Layer

400 g of 40/70 proppant sand was added to a KitchenAid mixer bowl. 16 g of SNF Flopam EM533 was added to the KitchenAid mixer bowl. The mixer was then turned on to the lowest setting and left to mix for 7 minutes. After mixing, the sand was split into 50 g samples and placed in a forced air oven at 80° C. for 1 hr. After drying, the modified proppant was screened through a 25 mesh sieve.

Example 17 Shear Stability Testing

Coated sand samples made in Examples 15 and 16 were tested for shear stability. 1 L of tap water was added to a square 1 L beaker. The beaker was then placed in an EC Engineering CLM4 paddle mixer. The mixer was set to mix at 300 rpm. Once mixing commenced, 50 g of the coated sand sample was added to the beaker. After 30 seconds of mixing at 300 rpm, the mixing was reduced to 200 rpm and continued for 20 minutes. At the end of this mixing, the mixture was poured into a 1 L graduated cylinder and allowed to settle. After 10 minutes, the settled bed height was recorded, as shown in Table 10. Higher bed heights indicate better proppant performance.

TABLE 10 Sand Sample Bed Height after shear, mm Untreated 40/70 Sand 13.24 Example 2 70.4 Example 3 57.64

Example 18 Brine Tolerance

Two 20 mL vials were filled with 10 mL of tap water. Separately, another two 20 mL vials were filled with 10 mL of a 1% KCl solution. 1 g of sand prepared in Example 15 was added to a vial containing tap water and 1 g was added to a vial containing 1% KCl. Also, 1 g of sand prepared in Example 6 was added to a vial containing tap water and 1 g was added to a vial containing 1% KCl. All four vials were inverted ˜7 times and then left to settle for 10 minutes. After settling, the bed heights were measured. The results are shown in Table 11.

TABLE 11 Sand Tap Water Bed Height, 1% KCl Bed Sample mm Height, mm Example 2 10.39 5.02 Example 6 17.15 9.23

Example 19 Abrasion Testing

Three 250 mL beakers were filled with 50 mL of tap water. One aluminum disk with a mass of about 5.5-6 g was placed in each of the beakers. One 2 inch stir bar was placed in each of the beakers as well. All three beakers were placed their own magnetic stir plates and the plates were set to speed setting 5. Six grams of 40/70 sand was added to one of the beakers. Six grams of sand prepared in Example 15 was placed a second beaker. The third beaker had no sand added at all. Each of the beakers was left to stir for 2 hours. After stirring, the aluminum disk was removed, washed and then dried. The mass was then measured again. The results, shown in Table 12, indicate that the sand prepared in Example 15 results in less abrasion to metal surfaces upon contact, compared with unmodified sand.

TABLE 12 Initial Total Mass, g Mass After 2 hrs, g Loss, g % Loss No Sand 5.62 5.612 0.008 0.14% 40/70 Sand, 6.044 6.027 0.017 0.28% uncoated Example 15 5.673 5.671 0.002 0.04% Sand

Example 20 Effect of Glycerol on Mixing

1 kg of dry 40/70 sand was added to the bowl of a KitchenAid stand mixer, model KSM90WH, which was fitted with the paddle attachment. 3.09 g of glycerol was mixed with 27.84 g of emulsion polymer then the mixture was added to the top of the sand and allowed to soak in for 1 minute. At time 0 the mixer was started at speed 4 (150 rpm primary rotation). Samples were collected at 1-2 minute intervals and dried for 1 hour at 90° C. Then, each sample was subjected to a shear test, where 50 g of SSP was added to 1 L of water and sheared at 550 s⁻¹ for 20 minutes. After settling for 10 min, the bed heights were recorded. The results of these shear tests are shown in FIG. 5. The graph demonstrates that both undermixing and overmixing can affect the behavior of the coated proppants, leading to dissociation of the polymer from the sand during the shear test. In this example, an optimal amount of mixing was between about 5 and 20 minutes. The effect of mixing duration upon performance suggests that the coating is fragile while wet, and it is more durable once it is dry. In comparison to the coating tests with emulsion polymer alone, coatings with glycerol-blended emulsions appeared to cause the processing window (i.e., the acceptable amount of mixing time) to widen. Additionally, glycerol-blended emulsion coatings appeared to invert more fully, leading to better coating properties such as increased bed heights.

Example 21 Production of Self-Suspending Proppant Using a Pug Mill

A 3 cubic foot pug mill type mixer was used to make a batch of self-suspending proppant. About 50 lbs of 40/70 mesh sand was added to the pug mill. In a 1 L beaker, about 756 g of SNF Flopam EM533 was added and 84 g of glycerol was mixed into the polymer. The entire mixture was then poured evenly on top of the sand in the pug mill. The pug mill was turned on and mixed at about 70 rpm. Samples were taken after 30, 60, 120, 180, 240, 300, 420, and 600 seconds of mixing. The samples were dried for one hour. After drying, the 50 g of each sample was added to 1 L of water and mixed in an EC Engineering CLM4 for 20 min at 200 rpm. After mixing, the sample was poured into a 1 L graduated cylinder and allowed to settle for 10 minutes. After settling, the bed height was measured. The results are shown in Table 13.

TABLE 13 Pug Mill Mixing Time (sec) Bed Height, mm 30 29.34 60 23.49 120 48.9 180 57.58 240 55.71 300 44.88 420 57.21 600 57.25

Example 22 Wet Aging

A 400 g sample of self-suspending proppant (SSP) was manufactured in the same manner as Example 15. The 400 g of SSP was split into 50 g samples and left in closed containers and left at room temperature. After drying for various amounts of time, the samples were tested in the same manner as Example 21. The results are shown in Table 14.

TABLE 14 Aging Time, hr Final Bed Height, mm 0 10.1 2 26.63 4 60.16

Example 23 SSP Plus Uncoated Proppant

10 mL of tap water was added to a 20 mL vial. Proppant sand, both SSP prepared in accordance with Example 15 and unmodified 40/70 was then added to the vial. The vial was inverted several times and then left to settle for 10 minutes. After settling, the bed height was measured. The results are shown in Table 15.

TABLE 15 SSP, grams 40/70 Sand, grams Settled Bed Height, mm 0.5 0.5 5.46 0.75 0.25 5.71 0.9 0.1 8.23

Example 24 Anti-Caking Agents Added to SSP

A 400 g batch of SSP was produced in the same manner as described in Example 15. The sample was split into about 50 g subsamples and then 0.25 g of fumed silica with an aggregate size of 80 nm was mixed into each sample. Samples were then covered and aged at room temperature. The samples were tested in the same manner as described in Example 21. The results are shown in Table 16.

TABLE 16 Hours Aging Settled Bed Height, mm 18 57.3 24 41.28 42 44.29 48 44.76 72 45.48

Example 25 Respirable Dust

200 g samples of uncoated and hydrogel-coated sand (40/70 mesh) prepared according to Example 15 were sieved with a 140 mesh screen, and the fine particulates that pass through the 140 mesh sieve were collected and weighed. The coated sample of sand demonstrated an 86% reduction on the amount of fine particulates relative to the uncoated sample of sand. The results are shown in Table 17.

TABLE 17 Weight of the Weight of Dust Percentage sample the dust of total Uncoated sample 200.011 g 0.0779 g 0.03895% Coated sample 200.005 g 0.0108 g 0.00540%

Example 26 Anti-Caking Agents with Different Particulate Size

50 g of 40/70 mesh sand was mixed with 2 g of SNF Flopam EM533 using the speed mixer for 30 seconds at 800 rpm. Then 0.625 g of an anti-caking agent was added and the material was again mixed in the speed mixer for 30 seconds. The samples were allowed to sit for 3 hours, then tested in a 20 min shear test, allowed to settle for 10 min and the bed height measured. Results are shown in Table 18. The anti-caking agents improved the bed height after shear testing with a wide range of particle sizes.

TABLE 18 Anti-caking agent Particulate Size Bed Height (mm) Talc (magnesium 12 microns 16.76 silicate) Calcium Silicate 1-3 microns 39.78 Fumed Silica (EH-5) 80 nanometers 73.87

Example 27 Chemical Composition of Anti-Caking Agents

A wide variety of anti-caking agents were tested, as listed in Table 19. For each agent tested, 700 g of 40/70 sand was mixed in the KitchenAid mixer at speed 1 (144 rpm) with 21.65 g of a 10% glycerol/90% EM533 mixture. 50 g samples were separated out and mixed with the appropriate amount of anti-caking agent in the speed mixer. Three samples, which were mixed with 1% calcium silicate, 1.5% diatomaceous earth, and 1.5% Kaolin respectively, were tested in the shear test immediately, while the other 7 samples were dried for 1 hour in an 80° C. oven along with a control sample with no anti-caking agent. All samples were tested in the same manner as Example 17. Table 19-A shows bed heights after shear testing wet (non-dried) samples with an anti-caking agent applied. Table 19-B shows bed height after shear testing of dried (1 hr at 80° C.) samples with anti-caking agent applied.

TABLE 19-A Anti-Caking Agent Amount Bed Height (mm) Calcium Silicate  0.5 g (1%) 30.26 Diatomaceous earth (DE) 0.75 g (1.5%) 12.95 Kaolin clay 0.75 g (1.5%) 18.46

TABLE 19-B Anti-Caking Agent Amount Bed Height (mm) NONE — 85.9 Sodium Bicarbonate 0.5 g (1%) 56.97 Cornstarch 0.5 g (1%) 32.29 Baby Powder (talc) 0.5 g (1%) 84.83 Dry-Floc AF (hydrophobic 0.5 g (1%) 32.24 starch) Tapioca Maltodextrin 0.5 g (1%) 27.08 Microcrystalline cellulose 0.5 g (1%) 40.12 Baking Powder 0.5 g (1%) 39.88

Example 28 Anti-Caking Agents: Amounts Needed for Drying

Seven 50 g samples of 40/70 sand were added to small plastic jars, followed by 2 g each of 10% glycerol/90% emulsion polymer mixture for each. After speed mixing for 30 seconds, 0.25 g, 0.375 g, 0.5 g, 0.675 g, 0.75 g, 1 g and 2.5 g of calcium silicate powder were added to the seven samples and the sand was again mixed for 30 seconds. The samples were shear tested without a further drying step, and the settled bed height was recorded in mm. The results are shown in FIG. 6. A similar experiment was carried out using silica as an anticaking agent. These tests showed that a sand coated with a hydrogel can be treated with an anticaking agent, yielding a product that does not require a separate drying step to produce an acceptable bed height after shear testing.

Example 29 Silica Anti-Caking Agents

50 g of 40/70 sand was added to a small jar, followed by 2 g of 10% glycerol/90% EM533. The jar was speed mixed at 800 rpm for 30 seconds, then the appropriate amount of fumed silica was added, as described in Table 20, and it was mixed for another 30 sec. The samples underwent a 20 min shear test and the bed heights were recorded. No oven drying was used. Results are shown in Table 20.

TABLE 20 Compound name Chemical character Amount added Bed Height EH-5 Amorphous fumed 1% 136.25 mm silica M-5 Untreated fumed silica 1% 123.52 mm TS-720 Treated fumed silica, 1%  26.21 mm siloxanes and silicones PG001 30% anionic colloidal 1% solids  15.30 mm silica, 25.9% solids

A batch of coated sand was mixed in the KitchenAid mixer and separated into several 50 g samples. Then 1 wt % of various sizes of fumed silica were added to each of 3 samples, mixed, and shear tested. These test results are shown in Table 21.

TABLE 21 Powder Approx. Size Amount added Bed height Aldrich Fumed  7 nm 1% 48.86 mm Silica Aldrich Silica 10 nm 1% 35.48 mm Nanopowder Cabot EH-5 80 nm aggregates 1% 59.10 mm

Example 30 Preheating Sand

500 g of 30/50 sand was placed in a 90° C. oven for 1 h with occasional stirring, until the temperature of the sand equilibrated. The sand was then mixed in a commercial planetary mixer until it reached the desired pre-heated temperature (45° C., 60° C. or 80° C.), at which point 20.8 g of the SNF Flopam EM533 was added and the sample mixed for 7 min. The batch was then divided and dried in the oven for a range of times at 80° C. For the non-preheated samples, 500 g of 30/50 sand was placed in the mixer bowl with 20.8 g of polymer emulsion, mixed for 7 minutes, and then dried for varying amounts of time. All samples were shear tested using the standard procedure: 50 g of sand added to 1000 g of tap water, stirred at a shear rate of 550 s⁻¹ for 20 minutes, then settled for 10 min in a 1 L graduated cylinder. The results are shown in FIG. 7. These results suggest that preheating the sand to 45° C. is acceptable but 60-80° C. results in lower bed height in shear tests.

Example 31 Forced Air Drying

50 g of 40/70 sand was mixed with of 4% emulsion polymer (2 g) prepared according to Example 14 using the speed mixer for 30 seconds. The sample was transferred to a container fitted with a hot air gun set at 90, 95 or 100° C. The sample was left under the heat gun for 30 min total, with 5 g samples taken out at the 5, 10, 15 and 30 min marks. These samples were then tested using the Small shear test, performed as follows: 100 mL of tap water was set stirring in a 300 mL beaker using a 2 inch stir bar spinning at 500 rpm; 5 g of the sand sample was added to the beaker and sheared for 3 minutes; the whole solution was transferred to a 100 mL graduated cylinder, inverted once, settled for 5 minutes, and the bed height measured. The results of these tests are shown in FIG. 8. As shown in the graph, higher temperatures of the incoming forced air caused more complete drying and better bed height. To test the susceptibility of SSP to shear while drying with forced air, a seven prong rake was pulled back and forth through the sample to simulate light shear while drying. Two 50 g batches of SSP were prepared and dried under 110° C. air for 30 min. The first was completely static, while the second was constantly raked during the 30 min dry time. Both samples were tested using the large shear test for 20 min with a settling time of 10 min. The sample with static drying gave a settled bed volume of 100.63 mm; while the sample dried with light shear gave a settled bed volume of 109.49 mm.

Example 32 Mixing with Vertical Screw

A small-scale vertical screw blender was constructed. Sand and SNF Flopam EM533 were added to the container, and then mixed with the screw turning at about 120 rpm. The sample was then split into two 50 g parts, one of which was oven dried at 80° C., the other mixed with 0.5 g (1 wt %) fumed silica. Both were then subjected to a shear test as described in Example 17. The results of bed height measurement were as follows: Oven Dried, 1 h gave a bed height of 101.34 mm; Undried, with 1% of 7 nm fumed silica added, gave a bed height of 91.47 mm. Both oven drying and the addition of anti-caking agent to dry the product produced high bed heights.

Example 33 Microwave Drying

50 g of 40/70 sand was added to a small plastic jar, and then mixed with 2 g of a blend containing (10% glycerol/90% emulsion polymer) in the speed mixer for 30 seconds at 800 rpm. The sample was placed in a 700 W microwave oven and heated on high for 45 seconds. The sample was sieved and cooled, then sheared at 200 rpm for 20 min in an EC Engineering CLM4 mixer. After mixing the sample was transferred to a 1 L beaker and left to settle for 10 minutes. After settling, the bed height was measured in millimeters, giving a bed height of 52.43 mm. Microwave heating gives acceptable bed heights with relatively short drying times.

Example 34 Mixing and Heating with Anti-Caking Agents

500 g 40/70 sand was mixed in a KitchenAid mixer with 20 g of (20% Glycerol/80% emulsion) for 8 min. Next was added 0.44% of Cabot EH-5 fumed silica and mixed for 2 minutes, and then the sample was heated with the heat gun. 50 g samples were collected at 13, 18, 24 and 26 minutes of mixing time. These were shear tested for 20 min and the bed heights recorded. The results are shown in FIG. 9. A combination of glycerol and silica made the processing window longer.

Example 35 Microwave Drying

400 g of 30/50 sand was combined with 16 g (4% wt) of emulsion polymer prepared according to Example 14 and mixed in a KitchenAid stand mixer for 7 minutes. One 50 g sample was dried using the oven (80° C.), and 7 other samples were placed in a 700 W microwave oven for 5, 10, 20, 30, 45, 60 and 120 seconds respectively. Shear tests (20 minutes long) as described in Example 12 and loss on ignition (LOI) tests were run on the samples. An LOI test consisted of adding 10 g of sand to a tared crucible, which was placed in a 960° C. oven for 1 hour. After heating for an hour, the crucible was cooled in a dessicator for 1 hour then weighed. Drying time, bed height and LOI are shown on Table 22. The difference between the initial and final weights was expressed as a percentage of the total initial sand weight, as shown in FIG. 10.

TABLE 22 Drying Method Drying time Bed height (mm) LOI (%) Oven  1 h 41.36 1.8 Microwave  5 sec 15.54 3.33 Microwave  10 sec 16.14 Microwave  20 sec 24.68 Microwave  30 sec 39.99 2.929 Microwave  45 sec 53.31 Microwave  60 sec 49.84 Microwave 120 sec 57.81 2.279

These tests suggest that the microwave drying technique removes predominantly the water, rather than the oil, from the coated samples.

Example 36 Vacuum Drying

250 g of 30/50 sand were combined with 10 g emulsion polymer as described in Example 14. The sand mixture was stirred in a KitchenAid stand mixer on lowest speed for 7 minutes, then separated into 50 g samples and dried in a vacuum oven under 24 inches Hg vacuum at 25° C., 50° C. and 85° C. for 1 hour, 1 hour, and 30 minutes respectively. The samples were cooled to room temperature, sieved, and shear tested (as described in Example 12) for 20 minutes. The results are shown in Table 23.

TABLE 23 Sample # Temperature (° C.) Time Bed Height (mm) 1 25  1 hour 16.79 2 50  1 hour 17.34 3 85 30 min 18.04

During these tests, none of the samples dried completely, although further testing may show that higher temperatures can effect more complete drying.

EQUIVALENTS

While specific embodiments of the subject invention have been disclosed herein, the above specification is illustrative and not restrictive. While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. Many variations of the invention will become apparent to those of skilled art upon review of this specification. Unless otherwise indicated, all numbers expressing reaction conditions, quantities of ingredients, and so forth, as used in this specification and the claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that can vary depending upon the desired properties sought to be obtained by the present invention. 

We claim:
 1. A modified proppant, comprising a proppant particle and a hydrogel coating, wherein the hydrogel coating is applied to a surface of the proppant particle and localizes on the surface to produce the modified proppant.
 2. The modified proppant of claim 1, wherein the hydrogel coating comprises a water-swellable polymer.
 3. The modified proppant of claim 1, wherein the hydrogel coating is applied to the surface as a liquid.
 4. The modified proppant of claim 3, wherein the hydrogel coating comprises a solvent or a carrier fluid, and wherein the liquid hydrogel coating becomes a dried hydrogel coating by removal of the solvent or the carrier fluid.
 5. The modified proppant of claim 4, wherein the dried hydrogel coating is capable of expanding in volume in contact with an aqueous fluid to form a swollen hydrogel coating having a thickness of at least about 10% greater than the dried hydrogel coating.
 6. The modified proppant of claim 1, wherein the hydrogel coating comprises a water-swellable polymer that responds to elevated temperatures or brine conditions by collapsing in volume or thickness.
 7. The modified proppant of claim 1, wherein the hydrogel coating comprises a hydrophobic comonomer selected from the group consisting of alkyl acrylate esters, N-alkyl acrylamides, N-isopropylacrylamide, propylene oxide, styrene, and vinylcaprolactam.
 8. The modified proppant of claim 1, wherein the hydrogel coating comprises a polymer selected from the group consisting of polyacrylamide, poly(acrylic acid), copolymers of acrylamide with acrylic acid salts, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, guar gum, carboxymethyl guar, carboxymethyl hydroxypropyl guar gum, hydrophobically associating swellable emulsion polymers, and latex polymers.
 9. The modified proppant of claim 1, wherein the hydrogel coating further comprises chemical additives selected from the group consisting of scale inhibitors, biocides, breakers, wax control agents, asphaltene control agents, and tracers.
 10. The modified proppant of claim 1, further comprising a cationic/anionic polymer pair comprising a cationic polymer and a high molecular weight anionic polymer.
 11. The modified proppant of claim 10, wherein the cationic polymer is selected from the group consisting of poly-DADMAC, LPEI, BPEI, chitosan, and cationic polyacrylamide.
 12. The modified proppant of claim 1, further comprising a crosslinking agent.
 13. The modified proppant of claim 12, wherein the crosslinking agent comprises a covalent crosslinker.
 14. The modified proppant of claim 13, wherein the covalent crosslinker comprises a functional group selected from the group consisting of an epoxide, an anhydride, an aldehyde, a diisocyanate, and a carbodiamide.
 15. The modified proppant of claim 13, wherein the covalent crosslinker is selected from the group consisting of polyethylene glycol, diglycidyl ether, epichlorohydrin, maleic anhydride, formaldehyde, glyoxal, glutaraldehyde, toluene diisocyanate, and methylene diphenyl diisocyanate, 1-ethyl-3-(3-dimethylaminopropyl)carbodiamide.
 16. The modified proppant of claim 1, further comprising a delayed hydration additive.
 17. The modified proppant of claim 16, wherein the delayed hydration additive is selected from the group consisting of a low hydrophilic-lipophilic balance surfactant, an exclusion agent capable of excluding a finishing surfactant, a light ionic crosslinking agent, a light covalent crosslinking agent and a monovalent salt charge shielder.
 18. The modified proppant of claim 1, further comprising an alcohol selected from the group consisting of ethylene glycol, propylene glycol, glycerol, propanol, and ethanol.
 19. The modified proppant of claim 1, further comprising an anticaking agent.
 20. A hydraulic fracturing formulation, comprising the modified proppant of claim
 1. 21. The formulation of claim 20, further comprising uncoated sand.
 22. The formulation of claim 20, further comprising fibers.
 23. A method of forming a modified proppant, comprising: providing a proppant particle; and applying a hydrogel coating to a surface of the proppant particle so that the hydrogel coating localizes on the surface.
 24. The method of claim 23, wherein the hydrogel coating is applied to the surface as a liquid.
 25. The method of claim 23, further comprising the step of drying the hydrogel coating on the surface by a drying process.
 26. The method of claim 25, wherein the hydrogel coating comprises a solvent or a carrier fluid, and wherein the hydrogel coating dries on the surface by removal of the solvent or the carrier fluid to form a dried hydrogel coating.
 27. The method of claim 25, wherein the step of drying comprises heating the hydrogel coating.
 28. The method of claim 26, further comprising exposing the dried hydrogel coating to an aqueous fluid to form a swollen hydrogel coating, wherein the swollen hydrogel coating expands in volume to have a thickness of at least about 10% greater than the thickness of the dried hydrogel coating.
 29. A method of fracturing a well, comprising: preparing the hydraulic fracturing formulation of claim 20, and introducing the hydraulic fracturing formulation into the well in an effective volume and at an effective pressure for hydraulic fracturing, thereby fracturing the well.
 30. A method of manufacturing a modified proppant, comprising: providing a proppant substrate particle and a fluid polymeric coating composition; applying the fluid polymeric coating composition on the proppant substrate particle; mixing the proppant substrate particle and the fluid polymer coating composition to form a modified proppant; and drying the modified proppant, wherein the fluid polymeric coating composition comprises a hydrogel polymer, and wherein the hydrogel polymer localizes on the surface of the proppant substrate particle to produce the modified proppant.
 31. The method of claim 30, wherein the manufacturing takes place at or near a point of use for the modified proppant.
 32. The method of claim 30, wherein the proppant substrate particle comprises sand.
 33. The method of claim 32, wherein the sand is obtained at or near the point of use for the modified proppant.
 34. The method of claim 30, further comprising adding an alcohol selected from the group consisting of ethylene glycol, propylene glycol, glycerol, propanol, and ethanol during or before the step of mixing the proppant substrate particles and the fluid polymer coating composition.
 35. The method of claim 30, further comprising adding an inversion promoter during or following the step of mixing the proppant substrate particles and the fluid polymer coating composition.
 36. The method of claim 30, further comprising adding an anticaking agent to the modified proppant. 